1. Field of the Invention
The present invention relates to an optical glass, a press-molding glass material comprised of the optical glass, an optical element comprised of the optical glass and a method of manufacturing the same, and a bonded optical element.
2. Discussion of the Background
Methods of manufacturing optical elements by heating and press molding glass materials can be roughly divided into the following two methods.
In the first method, a glass material that has been heated to a temperature at which its viscosity is about 103.5 to 104.5 dPa·s is introduced into a pressing mold and press molded. The molded article obtained is then ground and polished to manufacture an optical element. This method is referred to as the reheat pressing method.
In the second method, a glass material is heated to a temperature at which its viscosity is about 105 to 109 dPa·s and press molded to obtain an optical element. This method is referred to as the precision mold press molding method or the precision press molding method. In the second method, high pressure is applied to a high viscosity glass, and the shape of the molding surface of the pressing mold is precisely transferred to the glass, making it possible to form an optically functional surface without steps such as grinding or polishing. Thus, to prevent the molding surface of the mold from deteriorating with repeated press molding, glass with a low glass transition temperature is employed at a low press molding temperature.
As image pickup optical systems and projection optical systems with more functions and greater compactness have been developed in recent years, the need for optical elements made of high refractive index, high dispersion glass has increased. Document 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2004-161598) discloses a high refractive index, high dispersion glass for use in manufacturing such optical elements. The contents of Document 1 or English language family members US2003/191008A1, U.S. Pat. No. 6,828,265, US2004/220040A1, and U.S. Pat. No. 7,320,949 are expressly incorporated herein by reference in their entirety.
Combining a lens made of a high refractive index, high dispersion glass with a lens made of a fluorophosphate glass having both low dispersion and abnormal partial dispersion makes it possible to achieve good correction of chromatic aberration. In particular, a bonded lens obtained by bonding a lens made of a high refractive index, high dispersion glass with a lens made of a fluorophosphate glass can effectively increase the functionality and compactness of an optical system.
In a bonded lens, the bonded surfaces should be precisely bonded. To that end, the bonding surface of one of the lenses is desirably precisely polished into a convex surface and the bonding surface of the other lens into a concave surface. The reheat pressing method is better suited than the precision mold press molding method to the manufacturing of such a spherical polished lens. The precision mold press molding method is suited to the manufacturing of optical elements not suited to grinding and polishing, such as lenses with aspherical surfaces, but raises the cost of manufacturing optical elements that are suited to grinding and polishing, such as spherical lenses.
Accordingly, when the present inventors attempted to reheat press mold the glass disclosed in Document 1, a high refractive index, high dispersion glass, they found that it devitrified due to low thermal stability. Thus, the glass described in Document 1 is unsuitable as a glass material for producing bonded lenses by reheat press molding.